1. Field of the Invention
This invention relates in general to telecommunications, and more particularly to a method and apparatus for matching common mode output voltage at a switched-capacitor to continuous-time interface.
2. Description of Related Art
Global communications continues to demonstrate rapid growth rates. As more people become accustomed to the convenience of electronic mail, web-based facsimile transmission, electronic commerce, telecommuting and high-speed Internet access, the demand on the telecommunications industry to provide adequate bandwidth to provide this type of service also increases. The growth in the number of people using electronic communications will only increase as the price of Internet access and Internet access devices such as personal digital assistants (PDAs), computers, etc.
Today, copper telephone lines service almost all voice traffic and most of the Internet traffic. However, as content rich applications continue to grow, both public and private copper access networks are being challenged. The local portion of the enterprise becomes a major challenge for access providers. To take advantage of the increasingly popular innovations in telecommunications technology, additional telephone lines are being installed in private residences and businesses.
Although analog modems have managed to stretch their potential speed to 56 kilobits per second (kbps), small-office/home-office (SOHO) customers need far greater Internet bandwidth to accommodate multimedia applications ranging form three-dimensional web sites to video conferencing. Analog modems cannot deliver the necessary bandwidth and, therefore, have reached the end of their usefulness.
In response to these developments, communications companies are responding with a variety of digital access solutions, all variants of Digital Subscriber Line (DSL) technology. These DSL technologies differ dramatically in their abilities to address major SOHO applications and the requirements of telephone companies.
DSL technologies are transport mechanisms for delivering high-bandwidth digital data services via twisted-pair copper wires. These copper wires provide the cabling between the telephone company's central offices and subscribers. DSL technology is a copper loop transmission technology that solves the bottleneck problem often associated with the last mile between Network Service Providers and the users of those network services. DSL technology achieves broadband speeds over ordinary phone wire. While DSL technology offers dramatic speed improvements (up to 7+ Mbps) compared to other network access methods, the real strength of DSL-based services lies in the opportunities driven by multimedia applications required by today's network users, performance and reliability and economics.
Without such transport mechanisms, subscribers would have to rely on T1 (1.5 Mbps) or E1 (2.0 Mbps) service, which requires the phone company to install expensive new cabling to every location that wants high-speed digital service. The installation costs make T1/E1 service expensive. The original DSL service was ISDN DSL (ISDL), which was defined in the late 1980s. ISDL provides 160 kbps rates over a single twisted-pair at ranges up to 18,000 feet from the telephone company's central office. While this service has been deployed to may homes and small businesses all over the world, the demands of multimedia applications are already challenging IDSL's bandwidth.
Asymmetric Digital Service Line (ADSL) is currently being embraced by residential web surfers for its ability to quickly download music and video files. ADSL refers to modem technology that transforms twisted copper pair (ordinary phone lines) into a pipeline for ultra fast Internet access. As the name suggests, ADSL is not asynchronous transmission, but rather asymmetric digital transmission, i.e., ADSL transmits more than 6 Mbps (optionally up to 8 Mbps) to a subscriber, and as much as 640 kbps (optionally up to 1 Mbps) in the other direction.
ADSL has the ability to increase normal phone line capacity by 99% via a digital coding technique. This extra capacity means that one could simultaneously assess the World Wide Web and use the telephone or send a fax. A user of this technology could have uninterrupted Internet access that is always on-line. This technology also has the potential to be a cost-effective solution for residential customers, telecommuters and small business.
Still, there is a need for symmetric high-speed connection. For example, small businesses have become increasingly dependent on sophisticated voice and data products and services for competing against larger corporations. Until now, the cost of providing small businesses with professional telephony and data services was prohibitive. However, integrated access and virtual public branch exchanges (PBXs) are providing small businesses with voice mail, high-speed Internet access, multiple business lines and sufficient capabilities for telecommuters.
As mentioned above, symmetric services were traditionally delivered by T1 and E! lines. Within the DSL family, HDSL has long been used to provision T1 lines because its long reach requires regeneration-signal boosting-only every 12,000 feet, compared with every 4,000 feet for other T1 provisioning techniques. In fact, HDSL's ability to simplify and cheapen T1 deployment has made HDSL by far the most established of the DSL technology family.
As an inexpensive and flexible replacement for leased T1 lines, the HDSL2 standards are eagerly awaited by the DSL industry. HDSL2 replaces the aging HDSL standard that required two copper pairs. HDSL2 uses only one copper pair and is potentially rate adjustable. HDSL2, which is being developed within the framework of the American National Standards Institute (ANSI, New York), promises to make HDSL more compelling in two ways. While HDSL was a proprietary technique-modems at the central office (CO) and the customer premises had to come from the same vendor-HDSL2 will be an interoperable standard in which modems can be mixed. Perhaps the biggest selling point of HDSL2, however, is that it can use one pair of copper wires instead of HDSL's two. Network service providers thus have a choice. HDSL and one-pair HDSL2 have about the same reach, while two-pair HDSL2 adds as much as another 4,000 feet of reach, depending on the gauge of copper and other conditions. Hoping to propel the new DSL technology into the business arena, eight chip makers and OEMs have formed a consortium for the HDSL2 standard.
An HDSL2 transceiver includes a framer, a data pump and an analog interface for coupling to the twisted-pair line. In the transmit function, the framer accepts a digital signal and outputs to the data pump a serial digital signal that includes the data payload plus an HDSL2 overhead. In the receive function, the framer receives HDSL frames from the data pump.
The data pump includes a transceiver and an analog front end for receives the HDSL frames serially from the framer. The transceiver converts the HDSL frames into a transmit signal by first converting the HDSL frames into symbols. Typically, a modulator, such as a trellis code modulator (TCM) encodes the symbols into a pulse amplitude modulation (PAM) signal. The signal is further processed to condition and filter the PAM signal. The analog front end provides pulse shaping to analog signals. This process is reversed in the receive channel with echo cancellation provided to cancel most of the echoed transmit signal.
As mentioned, the analog front end includes a transmit and a receive channel. In the transmit channel, the analog front end receives a pulse width modulated signal stream from the transceiver. A switched-capacitor circuit filter shapes the transmitted signal to meet specific spectral templates. The receive channel consists of an automatic gain control (AGC) stage and an analog-to-digital (A/D) converter. The AGC stage sets the amplitude to the optimum level to prevent saturation of the A/D converter.
Switched-capacitor circuits are needed in communication applications to implement accurate on chip filters. Active implementation of these switched-capacitor circuit requires amplifiers with extremely high gain, and hence with high output impedance. A switched-capacitor stage using such an amplifier is not suitable for driving the output, which can be very low impedance. In such cases, the output of the switched-capacitor filter needs to be buffered using a continuous time unity gain buffer. This continuous time stage is required to have unity gain, high input impedance, low distortion and capability to drive low output impedance. Such a circuit stage requires matching of input common mode to the output common mode to keep signal distortion low through this stage.
The switched-capacitor filter stage uses a switched-capacitor common mode feedback circuit to implement the common mode control loop. This common mode feedback circuit has the advantage of low power, however the output common mode of the circuit can have noticeable offset from the common mode reference signal. This is due to charge injected on various bias nodes due to the switches. This error charge, and hence the offset can be minimized by increasing the size of the common mode feedback capacitor. This however increases the loading of the switched-capacitor amplifier and hence increases power consumption.
To solve this problem, a high input impedance unity gain buffer, with low noise and distortion performance, is implemented using differential circuit techniques for good common mode noise rejection. This differential circuit has its own output common mode control loop, which now is in continuous time domain. The unity gain stage is implemented with direct feedback from the output of the amplifier to the input of the amplifier, without any additional component. This allows the input of the amplifier to have the high input impedance of an MOS device, and hence not effect the amplifier gain of the previous switched-capacitor stage. However this also requires that the two input terminals (two for each side of the differential pair, total four terminals) of the amplifier track each other and have same DC level for appropriate biasing of the amplifier. Common mode errors can occur at the switched-capacitor and continuous time interface due to use of a switched-capacitor common mode feedback in one domain versus continuous time common mode feedback in another domain. Excessive offset between these terminals will result in distortion in this stage. Even when the common mode reference signal for both stage are same, there will be mismatches in these two common mode levels due to switched-capacitor related charge injection in the switched-capacitor common mode circuit, and this offset leads to distortion in the unity gain amplifier, which is undesirable.
It can be seen then that there is a need for a method and apparatus for matching common mode output voltage at a switched-capacitor to continuous-time interface.
It can also be seen then that there is a need for a method and apparatus for creating a continuous time measure of the common mode output of the switched-capacitor stage for use as the reference input for the continuous time stage. It can be seen then that there is a need for a method and apparatus for that tracks the output common mode of the output unity gain differential operational amplifier for eliminating common mode errors between the inputs of the amplifier and for reducing distortion in the output replica signal.